85 2.5sd will start but will die

[85 sd2.5]

it will sometimes start and run but when i knock the fuse block it will cut out and die i can hear some kind of clicking from the relay on top of the fuse block. also other day i was on highway and fuel gauge and heater fan would not work i am having a very hard time finding info on this truck until i found this forum i need to get this thing reliable so i can make it to work any help would be amazing our family is kind of depending on me fixing this i dont want to let them down cheers and thank you for any ideas

[plenzen]

As I do not have this model of truck I can only assume that there is a loose wire someplace along the way. The relay that you speak of may control the DPC module.
This may be a loose battery cable or perhaps a bad ground someplace. Check all the heavy gauge wires from the battery to ground and also to the starter that they are tight and have good contacts. As stated I am not certain how your truck is configured under the hood but there will be some means of getting the power to the "accessories" of the vehicle. A plug, or some such thing that will attached to the POS + side of the battery.

Paul